Excel 365 (Online) - User not able to edit unlocked cells on protected sheet

We have a team working on Excel files online (in the browser). The files:

• are stored on an internal SharePoint site
• accessed/edited in the browser (default is Chrome)
• have been made using Excel 365 desktop version and uploaded to SharePoint

I have one user who randomly cannot edit unlocked cells on a protected sheet, receiving the error message "This sheet is protected. Some parts may be view-only and can't be changed". The unlocked cells are surrounded by locked cells, and some of the unlocked cells are merged. The issue could appear when opening a file and then not be present when the file is opened again later. It is not specific to any particular file; it can happen to any file.

It's interesting to note that all other users with the same permissions can simultaneously work on these files without the issue, even while the user in question is experiencing the block/issue. Also, the user is able to edit cells with the same protection set up on other sheets, but could later be blocked on these same sheets when re-opening the file at another time. Recreating this issue is not consistent.

The cells locked/unlocked property has been set via 'Protection' tab in the 'Format Cells' dialog in Excel desktop. The sheet's protection has also been set in the desktop app.

I have tried:

• Removing and re-instating specific cell protection via the user's device when they have this issue present
• Un-merging protected cells and then re-enabling protection
• Using other devices with the user's login/account (Apple Mac and iPad) and browsers (Edge, Safari)
• Removing and re-adding the user to the required permissions group on the SharePoint site

Any ideas for why this particular user may be experiencing this issue?